DIY Battery Capacity Tester with Internal Resistance Measurement (Arduino)

The DIY battery capacity tester is an Arduino-based instrument that measures how much charge a rechargeable cell actually delivers, instead of trusting the capacity printed on its label. Many students and hobbyists own 18650 lithium-ion cells whose printed ratings are optimistic, reused, or simply unknown, so a cell that should run a project for hours can die far sooner than expected. This tester discharges one cell at a time through a controlled load, logs voltage and current throughout the discharge, and stops at a safe cutoff voltage. The result is a measured capacity figure in milliamp-hours plus an internal resistance estimate, both of which say far more about a cell's real condition than its label does. Because the build uses an Arduino Uno, a MOSFET-switched load, a shunt resistor and an OLED display, it also teaches measurement, power handling and firmware timing in one compact bench instrument.

The tester works by discharging the cell under test in a controlled way and measuring what comes out of it from start to finish. The cell is placed in its holder and its open-circuit voltage is recorded before any load is connected. The Arduino then switches on the MOSFET load, current flows through the power resistor, and the firmware samples voltage and current repeatedly, accumulating charge over time. At the same time it compares the loaded voltage with the earlier open-circuit reading to estimate internal resistance. The OLED display shows live voltage, current, elapsed time and accumulated capacity throughout the run. When the cell voltage reaches the cutoff level, the Arduino switches the load off and holds the final capacity and resistance figures on screen for recording. No separate lab instrument is needed for the core result, because the same Arduino that switches the load also performs the measurement and drives the display.

1. The charged cell is placed in the holder and the Arduino records its open-circuit voltage with no load connected.
2. The firmware switches the MOSFET on, connecting the power-resistor load, and discharge current begins to flow.
3. The Arduino repeatedly samples cell voltage and the voltage across the shunt resistor, averaging several readings to reduce noise.
4. Discharge current is computed from the shunt voltage, and the firmware integrates current over time to accumulate delivered capacity.
5. Internal resistance is derived from the drop between the open-circuit voltage and the loaded voltage, divided by the measured load current.
6. When the cell voltage reaches the cutoff voltage, the firmware switches the MOSFET off and holds the final capacity and resistance on the display.

Tech stack:

• Arduino Uno (ATmega328P)
• Arduino IDE firmware in C/C++
• MOSFET-switched power-resistor discharge load
• Shunt-resistor current sensing
• 0.96-inch OLED display over I2C
• 18650 lithium-ion cell holder
• Breadboard and plywood prototype base

The tester is built around four capabilities that work together during every discharge run. First, it performs a controlled discharge, switching the load on and off through a MOSFET while the Arduino tracks voltage and current. Second, it integrates discharge current over time to report delivered capacity in milliamp-hours, updating the figure live on the OLED display. Third, it estimates internal resistance from the voltage drop between the unloaded cell and the same cell under a known load current, a first-principles measurement rather than a guessed health percentage. Fourth, it protects the cell under test by stopping the discharge automatically at a safe cutoff voltage, so a forgotten test cannot over-discharge a lithium-ion cell. Together these give a student a repeatable bench procedure: charge the cell, run the tester, and record capacity and resistance for comparison.

• [Controlled MOSFET discharge] The Arduino switches the discharge load through a MOSFET, so each test starts and stops cleanly under firmware control instead of relying on a manually connected resistor. (Implemented)
• [Live OLED readout] Voltage, discharge current, elapsed time and accumulated capacity are shown live on a small OLED display throughout the run, with final figures held on screen when the test ends. (Implemented)
• [Capacity measurement in milliamp-hours] The firmware samples current at regular intervals and integrates it over time, so the reported capacity is the charge the cell actually delivered during that discharge. (Implemented)
• [Internal resistance estimation] The tester records the cell voltage before loading and again under a known load current, then derives resistance from the voltage drop divided by the load current. (Implemented)
• [Automatic cutoff protection] When the cell voltage reaches the configured cutoff, the firmware switches the load off automatically, so an unattended test cannot over-discharge the cell. (Implemented)
• [Adjustable cutoff and discharge current] The cutoff voltage and the discharge current are set as firmware constants, so they can be retuned for a different cell type or a gentler test without rewiring the prototype. (Configurable)
• [Serial logging and extra cell holders] A serial output of the test readings and holders for other cell sizes can be added on request, for students who want to plot discharge curves or test more than one cell format. (Optional customization, not in the base build)

• Working battery capacity tester prototype (Arduino, MOSFET load, shunt sensing, OLED display, cell holder)
• Arduino firmware source code with commented measurement and cutoff logic
• Circuit and wiring documentation for the discharge load and sense connections
• Component list with the controller, MOSFET, resistor, shunt and display used
• Step-by-step test procedure: charging, running a discharge, and recording capacity and resistance
• Project abstract PDF (10-section structure matching this page)
• Demonstration guidance for presenting a live discharge run